Slab-mantle interactions: 2. The formation of diamonds

Abstract

A wide range of evidence indicates that the formation of most diamonds was closely associated with the subduction of oceanic lithosphere into the mantle. Syngenetic minerals included in diamonds show that they crystallized in two distinct but spatially related lithologies-eclogite and refractory harzburgite/dunite-which together are characteristic of subducted lithosphere. The P-T regimes of diamond crystallization, as given by mineral inclusion thermobarometry, support this interpretation, as does the plastic deformation record of diamond. Most diamonds appear to have crystallized in the presence of a melt rather than a fluid phase. It is proposed that the formation of diamonds is another example of slab-mantle interaction. Water released by the dehydration of bodies of former serpentinite in the subducted lithosphere, initiates melting of the eclogitic oceanic crust. “Eclogitic” diamonds may precipitate from these melts within the subducted oceanic crust at depths of 150–400 km. Slab-derived melts also migrate into overlying depleted peridotite and undergo hybridisation with their new surroundings. “Peridotitic” diamonds may crystallize during the final stages of this process. The protolith in which “peridotitic” diamonds formed was an exceptionally refractory harzburgite or dunite that may have been residual after extraction of komatiitic magmas in the Archaean. Alternatively, it may have been produced via multi-stage melting processes in island-arc environments. Refractory protoliths so formed were transported beneath continents via subduction, thereby becoming incorporated in the roots of growing cratons. During subduction, the protolith underwent metamorphic re-equilibration, with olivine and chromian garnet being produced at the expense of orthopyroxene and chrome spinel. Many “peridotitic” diamonds crystallized over a narrow pressure interval around 6 GPa, apparently during shallow-angle subduction beneath continental regions. Refractory peridotite in the root of the craton has itself been subjected to another form of slab-mantle interaction independent of diamond formation, at relatively shallow levels. Siliceous fluids from subducted serpentinised (Archaean?) lithosphere have converted some olivine to orthopyroxene, thereby causing the wide dispersion of olivine/orthopyroxene ratios now observed in cratonic peridotite xenoliths.